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ASTRONOMY.

and the Lares, Plutarch , de Luna. This opinion was nothowever retained by his successors, in the Alexandrianschool. Contrary to the doctrine of the Greek philosophers,he taught that the stars were at different distances, andthat the orbit of the earth round the sun was an insensiblepoint, in comparison of the immense distance of the stars.The only work of Aristarchus which remains, is on themagnitude and distance of the sun and moon.

Very nearly contemporary with Aristarchus, was Euclid ,the geometrician of Alexandria: Manktiio, an astrologerand historian; and Aratus and Cleanthus, disciples ofZeno, the stoic philosopher, all of whom contributed tothe enlargement of astronomical knowledge; but particu-larly the two tirst named. Eratosthenes, bom atCyrene, 27<> years before Christ, succeeded Aristarchus ,being invited by Ptolemy Evergetes. This professor, is said,to have been the inventor of the Armillary sphere,an instrument or machine composed of moveable sides,representing the equator, the two colures, with the meridian;all of which turned round on an axis directed to thetwo poles of the w orld ; each of which circles wereanciently called armilla, and the whole machine astrolabus.This, however, is said to have been known long beforeEratosthenes; Diog. Laert. i. 119; he probably perfectedit. All instruments which could be contrived for the pro-motion of this divine science, were furnished at the publicexpense, and placed within the observatory of Alexandria.Assisted by these instruments, Eratosthenes tirst under-took to measure the obliquity of the ecliptics, or rather thedouble of that obliquity, that is, the distance from thetropics, which he made to be about forty-seven degrees ;the obliquity, or half of this distance, twenty-three degreesand a half. This grand attempt was to ascertain the exactdistance of a degree of the meridian, and thus to determinethe circumference of the earth; which he accomplishedwith wonderful exactness, considering the period in whichhe lived ; and he performed this by tire same method sinceadopted by the moderns who have succeeded him. He isalso reported to have discovered the true distance of thesun from the earth, Plutarch . Placit Phil. ii. 31.

The great Archimedes , lived contemporary with Era-tosthenes, that eminent geometrician of Syracuse , whose in-ventive genius in mechanics, had constructed engineswhich protracted the fall of that capital, with its island,Sicily, to the almost omnipotent power of Rome , for aconsiderable period. Likewise Conon, of Samos, who